The triangle inequality theorem states that the sum of any two sides of a triangle must be greater than the third side.
You can write this as a formula: A + B > x, where x is the third side.
A + B > x
4 + 5 > x
9 > x or x < 9
A + x > B
4 + x > 5
x > 1
B + x > A
5 + x > 4
x > -1
Given two side lengths 4 and 5, the third side must be between 1 and 9.